Pulse shape discrimination (PSD) provides means for high-energy neutron detection in the presence of gamma radiation background by utilizing the difference in the shapes of scintillation pulses excited by neutrons (recoil protons) and gamma (γ)-rays in organic scintillators. PSD phenomena are based on the existence of two-decay component fluorescence, in which, in addition to the main component decaying exponentially (prompt fluorescence), there is usually a slower emission that has the same wavelength, but longer decay time (delayed emission). According to a commonly accepted mechanism shown in FIG. 1, the fast component results from the direct radiative de-excitation of excited singlet states (S1), while the slow component originates from the collisional interaction of pairs of molecules (or excitons) in the lowest excited n-triplet states (T1).
Since the triplet is known to be mobile in some compounds, the energy migrates until two triplets collide and experience a process, shown as Equation 1:T1+T1→S0+S1  Equation 1
In Equation 1, T1 is a triplet, S0 is the ground state, and S1 is a first excited state. Finally, the delayed singlet emission occurs with a decay rate characteristic of the migration rate and concentration of the triplet population, which is represented as Equation 2:S1→S0+hv  Equation 2
In Equation 2, hv is fluorescence, while S0 is the ground state and S1 is a first excited state. The lifetime of the delayed emission is determined by the lifetime of T1 and the rate of T1T1 collisions. The short range of the energetic protons produced from neutron collisions yields a high concentration of triplets, compared to the longer range of the electrons from the gamma interactions, leading to the enhanced level of delayed emission with longer decay times in neutron-induced pulses in comparison to those produced by the gamma excitation.
The observation of PSD is believed to be, in part, related to the benzene ring structure, allowing for the migration of triplet energy. However, its relevance to other specific properties of organic materials, such as physical state, molecular and crystallographic structure, or presence of impurities is unknown.
It would be beneficial to obtain organic scintillator crystals, including those having stilbene, which exhibit good PSD and/or are useful for other applications.
It would also be beneficial to obtain organic scintillator crystals, including those having stilbene, and which do not exhibit PSD, but are useful nonetheless for neutron detection.